1. Field of the Invention
This invention relates to a safety device for medical electrical apparatus, and in particular to such apparatus for limiting current coupled to a body from an ultrasound transducer probe.
2. Brief Description of the Prior Art
Medical ultrasound probes tend to pick up electrical noise via capacitance coupled between the internal probe wiring and the body of the patient. This noise is particularly severe in probes designed for insertion into a body cavity (e.g., transesophageal probes, endocavity probes, laparoscopic probes, and catheter probes), because their contact surface area, and consequently their coupling capacitance, is larger than that in probes applied to the outside of the body.
The electrical noise is typically greatly reduced by adding an outer conductive ground shield to the insertable portion of the probe. The shield itself is insulated from the patient by a dielectric cover or housing and thus has its own coupling capacitance to the body. Often, the shield also adds mechanical strength to the probe, especially to a catheter probe. The shield is electrically connected to the ultrasound system chassis and prevents noise signals from degrading the ultrasound image by shunting them to system ground.
This noise reduction technique is well known and is effective in many situations, but has a major drawback. The coupling capacitance, whether it is associated with the internal probe wiring or with an outer ground shield, is also responsible for potentially dangerous leakage currents arising from a single-fault condition in the ultrasound system, or in another device already placed within the body. Underwriters Laboratories (UL) regulations require that the leakage current not exceed 20 .mu.A rms (see UL 544, Section 42.4.1).
The UL requirement is usually met by reducing the capacitive coupling between the chassis-grounded internal probe components and the patient. That is, the thickness of the insulating outer cover or housing is increased and/or the spacing between the internal current carrying components of the probe and the body is increased. This solution is difficult to implement in insertable probes, and particularly so in small diameter devices such as catheter probes where the insulation thickness must be kept to a minimum.
Prior art methods include:
1) Tolerate the coupling noise--do not compromise patient safety; PA1 2) Increase insulation thickness to reduce coupling capacitance--use an outer shield if possible; and PA1 3) Adjust the ultrasound system response to attenuate the frequency range containing the coupled noise. PA1 1) Electrical noise sources common in the hospital environment can be severe enough to render ultrasound imaging ineffective, misleading, or useless; and PA1 2) Probes can become unwieldy because of attempts to reduce the coupling capacitance by increasing the probe wall thickness. PA1 1) An active network that is transparent to ground current signals if they are lower than a certain prescribed threshold, and limits ground current signals which exceed the threshold. PA1 2) An active network (cf the network in U.S. PA1 3) An active network that is transparent to ground current signals having an alternating current (AC) content substantially above a predetermined cutoff frequency, but strongly limits ground current signals with AC content below the cutoff frequency. PA1 4) An active network that is transparent to ground current signals having an AC content exclusively above a predetermined cutoff frequency, but rapidly latches in an "open" state when it senses an appreciable ground current signal below its cutoff frequency.
Disadvantages of the prior art methods include: